This invention relates to a system for monitoring numerically controlled machines, and, more particularly, to a system for monitoring actual machining errors separately from following errors in individual control axes.
2. Description of the Prior Art
In the field of numerically controlled machines, there are presently available extremely accurate high reliability machines capable of following coded information from an input source. These machines are generally referred to as numerically controlled machine tools and are used for the generation and creation of machine parts having complex curves in more than one axis. Such parts typically require hours and days of machining to be completed. For example, the machining of a prototype landing gear strut for a new aircraft has required a period of time approximating 30 hours of machine time to make a part that had never been made before, and that the operator of the machine could neither imagine or visualize.
The expense of machining prototype parts and production parts involves not only the expense of the part itself, which in many cases approximates tens of thousands of dollars per item, but also includes the expenses of scheduling contracts, production lines and associated equipment, and the cost of rescheduling should the item being manufactured be faulty for any reason whatsoever.
In a typical system, the input device contains digital coded information describing the shape of the item being manufactured in an incremental form as measured from a suitable starting position. A machine control unit (MCU) specifically adapted to the operation of a given machine tool receives the digital coded input data and generates positional data in the form of squarewave signals calculated to move the machine tool along the path in space as defined by the input device. In other words, the MCU continually calculates where the cutting surface of the machine should be in space at any instant of time.
In the usual situation, the MCU generates the necessary positional data as X, Y and Z position command signals which operate separately to produce the cutting movement of machine tool slides in three mutually orthogonal directions, as determined by input data processing logic. The MCU actually generates a separate squarewave signal variable in phase for each of the different axes of the machine tool for a time corresponding to a given distance to achieve the tool motion defined by the input data as the vector sum of the three slide motions.
The individual output pulse trains of the MCU are fed to individual electronic servomechanisms, one for each axis. Each servomechanism receives position and rate feedback signals for the given axis from the machine tool. The position feedback is compared with the position command, and the difference, known as the "following error", is multiplied by an appropriate gain constant K and compared with the rate feedback. The difference, known as the command velocity, is fed to the prime mover for the one axis.
Information about the actual tool velocity contained in the rate feedback signal is usually not very accurate, or is very noisy. Consequently, the feedback control relied upon primarily, and sometimes exclusively, is position feedback. The servo control theory used is to control the tool by causing the drive velocity, V, of the prime mover to be proportional to the difference between the commanded position, S.sub.1, and the actual position, S.sub.2, of the tool: EQU V = K(S.sub.1 S.sub.2) = KE
where K is a gain constant and E = S.sub.1 -S.sub.2 is the "following error". Any deviation from this relationship constitutes a failure of the servomechanism, i.e., if the actual slide velocity deviates from the command velocity K(S.sub.1 -S.sub.2), there is a machining error.
Most numerically controlled machines use resolver feedback elements, or equivalent elements such as Inductosyn elements which employ a slider in place of a rotor, and which are referred to hereinafter generically as motion-to-phase-shift transducers, or more simply as transducers. Typically, such transducers are geared to 5 or 10 electrical revolutions (phase shifts) per inch of machine motion.
As noted hereinbefore, position command is represented by the phase of a command signal from the MCU, and actual position of the tool is represented by the phase of the transducer signal. The phase difference between these two signals represents the following error, E, in the foregoing equation. The characteristics of the machine tool dictate the gain, K. A typical value for a low gain servo is 1 IPM/MIL or V=1 inch per minute for 0.001 inch of the following error. Consequently, to provide a tool drive velocity, V, at 100 IPM requires 0.1 inch of following error which is not machining error.
Assuming numerical control systems are able to detect following error in increments of electrical revolutions of the transducers to 0.0001 inches of resolution, if the systems are set to stop only when the following error reaches some predetermined limit higher than could be encountered for the contour being cut, typically 0.3 to 0.4 inch, it would be possible for some malfunction to permit a following error below the limit to persist until tolerable machining error has been exceeded. In other words, if a servo loop should fail a following error of this limit must be detected before power to the tool is shut down, meaning that a machining error up to or even exceeding the limit of 0.3 to 0.4 inch will have already occurred. For example, consider a simple problem of cutting an inclined ramp in the Z direction. With the X and Y servo loops operating properly, but with total failure of the X axis slide, the machine will continue to operate until the Z axis following error exceeds the limit. At that time, the machine is stopped, but by then a substantial distance has been cut in the X and Y axis without any ramp in the Z direction. The defect in the part could require that it be scrapped.
In a copending application Ser. No. 342,247, now abandoned, titled IMPROVED RELIABILITY SYSTEM FOR MACHINE TOOLS, filed Mar. 16, 1973, by the same inventor and assigned to the same assignee, there is disclosed a system which employs position feedback sensors independent of servomechanism transducers for monitoring at all times the actual position of the cutting tool. The actual position data is then compared with commanded position data. However, aside from the disadvantage of requiring independent position transducers to be mounted on the machine tool, there is the disadvantage of having to make some calculations to determine the deviation of the actual tool position from its desired (commanded) position. It would be desirable to provide a relatively inexpensive monitoring system which could be provided as original equipment, or added as a retrofit to installed equipment for a wide variety of numerically controlled machine tools.